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JSP Scripting
Elements
Core Servlets & JSP book: www.coreservlets.com
More Servlets & JSP book: www.moreservlets.com
Servlet and JSP Training Courses: courses.coreservlets.com
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Slides © Marty Hall, http://www.coreservlets.com, book © Sun Microsystems Press
Agenda
•
•
•
•
•
•
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Basic syntax
Types of JSP scripting elements
Expressions
Predefined variables
Scriptlets
Declarations
Scripting Elements
www.coreservlets.com
Uses of JSP Constructs
Simple
Application
Complex
Application
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Scripting Elements
• Scripting elements calling servlet
code directly
• Scripting elements calling servlet
code indirectly (by means of
utility classes)
• Beans
• Custom tags
• Servlet/JSP combo (MVC),
with beans and possibly custom
tags
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Design Strategy: Limit Java
Code in JSP Pages
• You have two options
– Put 25 lines of Java code directly in the JSP page
– Put those 25 lines in a separate Java class and put 1 line
in the JSP page that invokes it
• Why is the second option much better?
– Development. You write the separate class in a Java
environment (editor or IDE), not an HTML environment
– Debugging. If you have syntax errors, you see them
immediately at compile time. Simple print statements can
be seen.
– Testing. You can write a test routine with a loop that does
10,000 tests and reapply it after each change.
– Reuse. You can use the same class from multiple pages.
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Scripting Elements
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Basic Syntax
• HTML Text
– <H1>Blah</H1>
– Passed through to client. Really turned into servlet code
that looks like
• out.print("<H1>Blah</H1>");
• HTML Comments
– <!-- Comment -->
– Same as other HTML: passed through to client
• JSP Comments
– <%-- Comment --%>
– Not sent to client
• To get <% in output, use <\%
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Scripting Elements
www.coreservlets.com
Types of Scripting Elements
• Expressions
– Format: <%= expression %>
– Evaluated and inserted into the servlet’s output. I.e.,
results in something like out.print(expression)
• Scriptlets
– Format: <% code %>
– Inserted verbatim into the servlet’s _jspService method
(called by service)
• Declarations
– Format: <%! code %>
– Inserted verbatim into the body of the servlet class,
outside of any existing methods
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Scripting Elements
www.coreservlets.com
JSP Expressions
• Format
– <%= Java Expression %>
• Result
– Expression evaluated, converted to String, and placed
into HTML page at the place it occurred in JSP page
– That is, expression placed in _jspService inside out.print
• Examples
– Current time: <%= new java.util.Date() %>
– Your hostname: <%= request.getRemoteHost() %>
• XML-compatible syntax
– <jsp:expression>Java Expression</jsp:expression>
– XML version not supported by Tomcat 3. Until JSP 1.2,
servers are not required to support it. Even then, you
cannot mix versions within a single page.
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Scripting Elements
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JSP/Servlet Correspondence
• Original JSP
<H1>A Random Number</H1>
<%= Math.random() %>
• Possible resulting servlet code
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
HttpSession session = request.getSession(true);
JspWriter out = response.getWriter();
out.println("<H1>A Random Number</H1>");
out.println(Math.random());
...
}
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Scripting Elements
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Example Using JSP
Expressions
<BODY>
<H2>JSP Expressions</H2>
<UL>
<LI>Current time: <%= new java.util.Date() %>
<LI>Your hostname: <%= request.getRemoteHost() %>
<LI>Your session ID: <%= session.getId() %>
<LI>The <CODE>testParam</CODE> form parameter:
<%= request.getParameter("testParam") %>
</UL>
</BODY>
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Scripting Elements
www.coreservlets.com
Predefined Variables
• request
– The HttpServletRequest (1st argument to service/doGet)
• response
– The HttpServletResponse (2nd arg to service/doGet)
• out
– The Writer (a buffered version of type JspWriter) used to
send output to the client
• session
– The HttpSession associated with the request (unless
disabled with the session attribute of the page directive)
• application
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– The ServletContext (for sharing data) as obtained via
getServletContext().
www.coreservlets.com
Scripting Elements
JSP Scriptlets
• Format
– <% Java Code %>
• Result
– Code is inserted verbatim into servlet's _jspService
• Example
– <%
String queryData = request.getQueryString();
out.println("Attached GET data: " + queryData);
%>
– <% response.setContentType("text/plain"); %>
• XML-compatible syntax
– <jsp:scriptlet>Java Code</jsp:scriptlet>
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Scripting Elements
www.coreservlets.com
JSP/Servlet Correspondence
• Original JSP
<%= foo() %>
<% bar(); %>
• Possible resulting servlet code
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
HttpSession session = request.getSession(true);
JspWriter out = response.getWriter();
out.println(foo());
bar();
...
}
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Scripting Elements
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Example Using JSP Scriptlets
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Color Testing</TITLE>
</HEAD>
<%
String bgColor = request.getParameter("bgColor");
boolean hasExplicitColor;
if (bgColor != null) {
hasExplicitColor = true;
} else {
hasExplicitColor = false;
bgColor = "WHITE";
}
%>
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Scripting Elements
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Example Using JSP Scriptlets
(Continued)
<BODY BGCOLOR="<%= bgColor %>">
<H2 ALIGN="CENTER">Color Testing</H2>
<%
if (hasExplicitColor) {
…
} else {
…
}
%>
</BODY>
</HTML>
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Scripting Elements
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JSP Scriptlets: Results
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Scripting Elements
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Using Scriptlets to Make Parts
of the JSP File Conditional
• Point
– Scriplets are inserted into servlet exactly as written
– Need not be complete Java expressions
– Complete expressions are usually clearer and easier to
maintain, however
• Example
– <% if (Math.random() < 0.5) { %>
Have a <B>nice</B> day!
<% } else { %>
Have a <B>lousy</B> day!
<% } %>
• Representative result
– if (Math.random() < 0.5) {
out.println("Have a <B>nice</B> day!");
} else {
out.println("Have a <B>lousy</B> day!");
}
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Scripting Elements
www.coreservlets.com
JSP Declarations
• Format
– <%! Java Code %>
• Result
– Code is inserted verbatim into servlet's class definition,
outside of any existing methods
• Examples
– <%! private int someField = 5; %>
– <%! private void someMethod(...) {...} %>
• Design consideration
– Fields are clearly useful. For methods, it is usually better
to define the method in a separate Java class.
• XML-compatible syntax
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– <jsp:declaration>Java Code</jsp:declaration>
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Scripting Elements
JSP/Servlet Correspondence
• Original JSP
<H1>Some Heading</H1>
<%!
private String randomHeading() {
return("<H2>" + Math.random() + "</H2>");
}
%>
<%= randomHeading() %>
• (Alternative: make randomHeading a static
method in a separate Java class)
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Scripting Elements
www.coreservlets.com
JSP/Servlet Correspondence
• Possible resulting servlet code
public class xxxx implements HttpJspPage {
private String randomHeading() {
return("<H2>" + Math.random() + "</H2>");
}
public void _jspService(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
HttpSession session = request.getSession(true);
JspWriter out = response.getWriter();
out.println("<H1>Some Heading</H1>");
out.println(randomHeading());
...
} ...
}
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Scripting Elements
www.coreservlets.com
Example Using
JSP Declarations
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD><TITLE>JSP Declarations</TITLE>
<LINK REL=STYLESHEET
HREF="JSP-Styles.css"
TYPE="text/css">
</HEAD>
<BODY>
<H1>JSP Declarations</H1>
<%! private int accessCount = 0; %>
<H2>Accesses to page since server reboot:
<%= ++accessCount %></H2>
</BODY>
</HTML>
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Scripting Elements
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JSP Declarations: Result
• After 15 total visits by an arbitrary number
of different clients
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Scripting Elements
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JSP Declarations: the
jspInit and jspDestroy Methods
• JSP pages, like regular servlets, sometimes
want to use init and destroy
• Problem: the servlet that gets built from the
JSP page might already use init and destroy
– Overriding them would cause problems.
– Thus, it is illegal to use JSP declarations to declare
init or destroy.
• Solution: use jspInit and jspDestroy.
– The auto-generated servlet is guaranteed to call these
methods from init and destroy, but the standard versions
of jspInit and jspDestroy are empty (placeholders for you
to override).
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Scripting Elements
www.coreservlets.com
JSP Declarations and
Predefined Variables
• Problem
– The predefined variables (request, response, out, session,
etc.) are local to the _jspService method. Thus, they are
not available to methods defined by JSP declarations or to
methods in helper classes. What can you do about this?
• Solution: pass them as arguments. E.g.
<%!
private void someMethod(HttpSession s) {
doSomethingWith(s);
}
%>
<% someMethod(session); %>
• Note that the println method of JspWriter
throws IOException
– Use “throws IOException” for methods that use println
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Scripting Elements
www.coreservlets.com
Using JSP Expressions as
Attribute Values
• Static Value
– <jsp:setProperty
name="author"
property="firstName"
value="Marty" />
• Dynamic Value
– <jsp:setProperty
name="user"
property="id"
value=’<%= "UserID" + Math.random() %>’ />
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Scripting Elements
www.coreservlets.com
Attributes That Permit
JSP Expressions
• The name and value properties of
jsp:setProperty
– See upcoming section on beans
• The page attribute of jsp:include
– See upcoming section on including files and applets
• The page attribute of jsp:forward
– See upcoming section on integrating servlets and JSP
• The value attribute of jsp:param
– See upcoming section on including files and applets
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Scripting Elements
www.coreservlets.com
Summary
• JSP Expressions
– Format: <%= expression %>
– Wrapped in out.print and inserted into _jspService
• JSP Scriptlets
– Format: <% code %>
– Inserted verbatim into the servlet’s _jspService method
• JSP Declarations
– Format: <%! code %>
– Inserted verbatim into the body of the servlet class
• Predefined variables
– request, response, out, session, application
• Limit the Java code that is directly in page
– Use helper classes, beans, custom tags, servlet/JSP combo
26
Scripting Elements
www.coreservlets.com
Questions?
Core Servlets & JSP book: www.coreservlets.com
More Servlets & JSP book: www.moreservlets.com
Servlet and JSP Training Courses: courses.coreservlets.com
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Slides © Marty Hall, http://www.coreservlets.com, book © Sun Microsystems Press